The present invention relates to methods and apparatus for detecting particular frequencies of vibration, and especially for detecting and selecting particular frequencies of vibration using detection and selection apparatus comprising piezoelectrically-tunable beam members such as cantilevers and very small cantilevers, often called microcantilevers.
The resonance frequencies of a beam occur at discrete values based on the geometrical and mechanical properties of the beam and the environment in which it is located. The efficiency of resonance is measured by the quality factor (or Q-factor), where large Q-factors correspond to high efficiency. High-Q beams such as cantilever beams can be used as efficient listening devices for particular frequencies, with much higher sensitivity and specificity for particular acoustic bands of interest in comparison to conventional acoustic transducers. Moreover, microcantilevers, which are only a few hundred microns in length, are also much more simple to produce and could be far smaller in comparison to standard microphone technologies. Unfortunately, as an inevitable consequence of their high specificity, one would need an exorbitant number of fixed-frequency cantilevers to cover a broad frequency spectrum. Because of this simple reason cantilever-based listening devices have not attracted significant attention. Thus, it is desirable to make a high-Q cantilever that uses a piezoelectric method to achieve broad frequency tunability. The resonance frequency of such a cantilever can be changed by varying a voltage applied to a piezoelectric layer and thereby varying a stress within a stress-sensitive material which may comprise an element of the cantilever.
Accordingly, it is an object of the present invention to provide a new and improved method and apparatus for tuning the resonant frequency of a beam such as a cantilever element, and more specifically to provide a new method and apparatus for tuning the resonant frequency of a beam such as a cantilever element by varying a voltage applied to a piezoelectric material which comprises the beam or an element thereof. Further and other objects of the present invention will become apparent from the description contained herein.
In accordance with one aspect of the present invention, the foregoing and other objects are achieved by a piezoelectrically-tunable beam for detecting a particular frequency of vibration which comprises: a beam element having an end and a surface, and being fixedly disposed on the end; a stress-sensitive means for controlling stiffness of the beam selected from the group consisting of: a stress sensitive coating having a stiffness that varies with the stress therein affixed on the surface of the beam element and the beam element material having a stiffness that varies with the stress therein; a piezoelectric means for varying stiffness selected from the group consisting of a piezoelectric coating having stiffness that varies with a voltage applied thereacross affixed on the surface of the beam element and the beam element material having stiffness that varies with an electrical potential applied thereacross; and an electrical potential means suitably disposed and connected for providing an electrical potential across the piezoelectric means so that the electrical potential causes bond lengths between constituent molecules within the piezoelectric means to vary, and thus produces a change in stress in the stress sensitive means and a change in the resonance frequency of the piezoelectrically-tunable beam.
In accordance with a second aspect of the present invention, the foregoing and other objects are achieved by a method for detecting a particular frequency of acoustical vibration in an environment, the method comprising the steps of: providing a piezoelectrically-tunable beam comprising a beam element having an end and a surface and being fixedly disposed on the end; a stress-sensitive means for controlling stiffness of the beam selected from the group consisting of: a stress sensitive coating having a stiffness that varies with the stress therein affixed on the surface of the beam element and the beam element material having a stiffness that varies with the stress therein; a piezoelectric means for varying stiffness selected from the group consisting of: a piezoelectric coating having stiffness that varies with an electrical potential applied thereacross affixed on the surface of the beam element and the beam element material having stiffness that varies with an electrical potential applied thereacross; and electrical potential means suitably disposed and connected for providing an electrical potential across the piezoelectric means; exposing the beam element to the environment; activating the electrical potential means to cause an electrical potential across the piezoelectric means to cause bond lengths between constituent molecules in the piezoelectric means to vary thereby producing a change in stress in the stress sensitive means and a change in the resonance frequency of the piezoelectrically-tunable beam to establish the resonance frequency of the piezoelectrically-tunable beam at the desired frequency; and determining from the resonance response of the piezoelectrically-tunable beam whether the particular frequency of vibration is detected. In accordance with a third aspect of the present invention, the foregoing and other objects are achieved by a method for selecting a desired frequency of acoustical vibration from a mixture of frequencies which comprises the steps of: providing a piezoelectrically-tunable beam comprising a beam element having an end and a surface and being fixedly disposed on the end; a stress-sensitive means for controlling stiffness of the beam selected from the group consisting of: a stress sensitive coating having a stiffness that varies with the stress therein affixed on the surface of the beam element and the beam element material having a stiffness that varies with the stress therein; a piezoelectric means for varying stiffness selected from the group consisting of: a piezoelectric coating having stiffness that varies with an electrical potential applied thereacross affixed on the surface of the beam element and the beam element material having stiffness that varies with an electrical potential applied thereacross; and electrical potential means suitably disposed and connected for providing an electrical potential across the piezoelectric means; activating the electrical potential means to cause an electrical potential across the piezoelectric means to cause bond lengths between constituent molecules in the piezoelectric means to vary thereby producing a change in stress in the stress sensitive means and a change in the resonance frequency of the piezoelectrically-tunable beam to establish the resonance frequency of the piezoelectrically-tunable beam at the desired frequency; exposing the piezoelectrically-tunable beam to the mixture of frequencies; and determining from the resonance response of the piezoelectrically-tunable beam whether the particular frequency of vibration is detected.